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Abstract:

An electrosurgical generator for supplying electrosurgical energy to
tissue is disclosed. The generator includes sensor circuitry configured
to measure at least one tissue or energy parameter and a controller
configured to generate a plot of the at least one tissue or energy
parameter including a plurality of tissue parameter values, wherein the
controller is further configured to normalize the plot of the at least
one tissue or energy parameter with respect to treatment volume.

Claims:

1. An electrosurgical generator for supplying electrosurgical energy to
tissue, comprising: sensor circuitry configured to measure at least one
tissue or energy parameter; and a controller configured to generate a
plot of the at least one tissue or energy parameter including a plurality
of tissue parameter values, wherein the controller is further configured
to normalize the plot of the at least one tissue or energy parameter with
respect to treatment volume.

2. The electrosurgical generator according to claim 1, wherein the at
least one tissue or energy parameter is selected from the group
consisting of imaginary impedance, real impedance, phase angle, voltage,
current and average power.

3. The electrosurgical generator according to claim 1, regulate output of
the electrosurgical generator based on the normalized plot of the at
least one tissue or energy parameter.

4. The electrosurgical generator according to claim 1, wherein the
controller is further configured to filter the plot of the at least one
tissue or energy parameter to form a filtered plot of the at least one
tissue or energy parameter.

5. The electrosurgical generator according to claim 4, wherein the
controller is further configured to execute at least two recursive
filters configured to recursively process the plot of the at least one
tissue or energy parameter.

6. The electrosurgical generator according to claim 1, wherein the
controller is configured to regulate the electrosurgical generator based
on a rate of change of the at least one tissue or energy parameter and is
configured to control the generator to continue application of the
electrosurgical energy when the rate of change of the at least one tissue
or energy parameter is above a first predetermined threshold.

7. The electrosurgical generator according to claim 6, wherein the
controller is configured to regulate the electrosurgical generator to
discontinue application of the electrosurgical energy when the rate of
change of the at least one tissue or energy parameter is below a second
predetermined threshold.

8. The electrosurgical generator according to claim 7, wherein the
controller is configured to regulate the electrosurgical generator to
commence intelligent shut-off of the electrosurgical energy for a
duration of a predetermined time delay when the rate of change of the at
least one tissue or energy parameter is between the first and second
predetermined thresholds.

9. The electrosurgical generator according to claim 8, wherein the
controller is configured to regulate the electrosurgical generator to
restart application of the electrosurgical energy when the rate of change
of the at least one tissue or energy parameter is above the first
predetermined threshold during the time delay.

10. A method for supplying electrosurgical energy to tissue, comprising
the steps of: measuring at least one tissue or energy parameter;
generating a plot of the at least one tissue or energy parameter
including a plurality of tissue parameter values; normalizing the plot of
the at least one tissue or energy parameter with respect to treatment
volume; and regulating output of the electrosurgical generator based on
the normalized plot of the at least one tissue or energy parameter.

11. The method according to claim 10, wherein the at least one tissue or
energy parameter is selected from the group consisting of imaginary
impedance, real impedance, phase angle, voltage, current and average
power.

12. The method according to claim 10, further comprising the step of:
recursively processing the plot of the at least one tissue or energy
parameter to form a filtered plot of the at least one tissue or energy
parameter.

13. The method according to claim 12, wherein the regulating further
includes measuring a rate of change of the imaginary impedance of tissue
and the regulating step further includes the step of continuing
application of the electrosurgical energy when the rate of change of the
imaginary impedance is above a first predetermined threshold.

14. The method according to claim 13, wherein the regulating step further
includes the step of: discontinuing application of the electrosurgical
energy when the rate of change of the imaginary impedance is below a
second predetermined threshold.

15. The method according to claim 14, wherein the regulating step further
includes comprising the step of: commencing intelligent shut-off of the
electrosurgical energy for a duration of predetermined time delay when
the rate of change of the imaginary impedance is between the first and
second predetermined thresholds.

16. The method according to claim 15, wherein the regulating step further
includes the step of: restarting application of the electrosurgical
energy when the rate of change of the imaginary impedance is above the
first predetermined threshold during the time delay.

17. A method for supplying electrosurgical energy to tissue, comprising
the steps of: measuring at least one tissue or energy parameter;
generating a plot of the at least one tissue or energy parameter
including a plurality of tissue parameter values; filtering the plot of
the at least one tissue or energy parameter to form a filtered plot of
the at least one tissue or energy parameter; normalizing the filtered
plot of the at least one tissue or energy parameter with respect to
treatment volume; and regulating output of the electrosurgical generator
based on the normalized plot of the at least one tissue or energy
parameter.

18. The method according to claim 17, wherein the at least one tissue or
energy parameter is selected from the group consisting of imaginary
impedance, real impedance, phase angle, voltage, and average power.

19. The method according to claim 17, wherein the filtering further
includes recursively processing the plot of the at least one tissue or
energy parameter to form the filtered plot of the at least one tissue or
energy parameter.

20. The method according to claim 17, wherein the regulating further
includes: measuring a rate of change of the imaginary impedance of tissue
and the regulating step further includes the step of continuing
application of the electrosurgical energy when the rate of change of the
imaginary impedance is above a first predetermined threshold;
discontinuing application of the electrosurgical energy when the rate of
change of the imaginary impedance is below a second predetermined
threshold; commencing intelligent shut-off of the electrosurgical energy
for a duration of predetermined time delay when the rate of change of the
imaginary impedance is between the first and second predetermined
thresholds; and restarting application of the electrosurgical energy when
the rate of change of the imaginary impedance is above the first
predetermined threshold during the time delay.

Description:

BACKGROUND

[0001] 1. Technical Field

[0002] The present disclosure relates to electrosurgical apparatuses,
systems and methods. More particularly, the present disclosure is
directed to electrosurgical systems and methods for monitoring
electrosurgical procedures and intelligent termination thereof based on
various sensed tissue parameters.

[0003] 2. Background of Related Art

[0004] Energy-based tissue treatment is well known in the art. Various
types of energy (e.g., electrical, ohmic, resistive, ultrasonic,
microwave, cryogenic, laser, etc.) are applied to tissue to achieve a
desired result. Electrosurgery involves application of radio frequency
electrical current to a surgical site to cut, ablate, coagulate or seal
tissue. In monopolar electrosurgery, a source or active electrode
delivers radio frequency energy from the electrosurgical generator to the
tissue and a return electrode carries the current back to the generator.
In monopolar electrosurgery, the source electrode is typically part of
the surgical instrument held by the surgeon that is applied to the
tissue. A patient return electrode is placed remotely from the active
electrode to carry the current back to the generator.

[0005] Ablation is most commonly a monopolar procedure that is
particularly useful in the field of cancer treatment, where one or more
RF ablation needle electrodes that (usually of elongated cylindrical
geometry) are inserted into a living body. A typical form of such needle
electrodes incorporates an insulated sheath disposed over an exposed
(uninsulated) tip. When the RF energy is provided between the return
electrode and the inserted ablation electrode, RF current flows from the
needle electrode through the body. Typically, the current density is very
high near the tip of the needle electrode, which tends to heat and
destroy surrounding issue.

[0006] In bipolar electrosurgery, one of the electrodes of the hand-held
instrument functions as the active electrode and the other as the return
electrode. The return electrode is placed in close proximity to the
active electrode such that an electrical circuit is formed between the
two electrodes (e.g., electrosurgical forceps). In this manner, the
applied electrical current is limited to the body tissue positioned
between the electrodes. When the electrodes are sufficiently separated
from one another, the electrical circuit is open and thus inadvertent
contact with body tissue with either of the separated electrodes prevents
the flow of current.

[0007] Bipolar electrosurgical techniques and instruments can be used to
coagulate blood vessels or tissue, e.g., soft tissue structures, such as
lung, brain and intestine. A surgeon can either cauterize,
coagulate/desiccate and/or simply reduce or slow bleeding, by controlling
the intensity, frequency and duration of the electrosurgical energy
applied between the electrodes and through the tissue. In order to
achieve one of these desired surgical effects without causing unwanted
charring of tissue at the surgical site or causing collateral damage to
adjacent tissue, e.g., thermal spread, it is necessary to control the
output from the electrosurgical generator, e.g., power, waveform,
voltage, current, pulse rate, etc.

[0008] It is known that measuring the electrical impedance and changes
thereof across the tissue at the surgical site provides a good indication
of the state of desiccation or drying of the tissue, e.g., as the tissue
dries or loses moisture, the impedance across the tissue rises. This
observation has been utilized in some electrosurgical generators to
regulate the electrosurgical power based on measured tissue impedance.

SUMMARY

[0009] An electrosurgical generator for supplying electrosurgical energy
to tissue is disclosed. The generator includes sensor circuitry
configured to measure at least one tissue or energy parameter and a
controller configured to generate a plot of the at least one tissue or
energy parameter including a plurality of tissue parameter values,
wherein the controller is further configured to normalize the plot of the
at least one tissue or energy parameter with respect to treatment volume.

[0010] According to an embodiment of the present disclosure, a method for
supplying electrosurgical energy to tissue is disclosed. The method
includes: measuring at least one tissue or energy parameter; generating a
plot of the at least one tissue or energy parameter including a plurality
of tissue parameter values; normalizing the plot of the at least one
tissue or energy parameter with respect to treatment volume; and
regulating output of the electro surgical generator based on the
normalized plot of the at least one tissue or energy parameter.

[0011] A method for supplying electrosurgical energy to tissue is also
contemplated by the present disclosure. The method includes measuring at
least one tissue or energy parameter; generating a plot of the at least
one tissue or energy parameter including a plurality of tissue parameter
values; filtering the plot of the at least one tissue or energy parameter
to form a filtered plot of the at least one tissue or energy parameter;
normalizing the filtered plot of the at least one tissue or energy
parameter with respect to treatment volume; and regulating output of the
electrosurgical generator based on the normalized plot of the at least
one tissue or energy parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Various embodiments of the present disclosure are described herein
with reference to the drawings wherein:

[0013]FIG. 1A is a schematic block diagram of a monopolar electrosurgical
system according to one embodiment of the present disclosure;

[0014]FIG. 1B is a schematic block diagram of a bipolar electrosurgical
system according to one embodiment of the present disclosure;

[0015]FIG. 2 is a schematic block diagram of a generator according to an
embodiment of the present disclosure;

[0016]FIG. 3 is a plot of treatment volume during application of
electrosurgical energy according to one embodiment of the present
disclosure;

[0017]FIG. 4 is a plot of treatment volume and impedance during
application of electrosurgical energy according to one embodiment of the
present disclosure;

[0018]FIG. 5 is a plot of phase angle during application of
electrosurgical energy according to one embodiment of the present
disclosure;

[0019]FIG. 6 is a plot of real impedance during application of
electrosurgical energy according to one embodiment of the present
disclosure;

[0020] FIG. 7 is a plot of voltage during application of electrosurgical
energy according to one embodiment of the present disclosure;

[0021] FIG. 8 is a plot of average power during application of
electrosurgical energy according to one embodiment of the present
disclosure; and

[0022]FIG. 9 is a flow chart of a method according to one embodiment of
the present disclosure.

DETAILED DESCRIPTION

[0023] Particular embodiments of the present disclosure are described
hereinbelow with reference to the accompanying drawings. In the following
description, well-known functions or constructions are not described in
detail to avoid obscuring the present disclosure in unnecessary detail.

[0024] The generator according to the present disclosure can perform
monopolar and bipolar electrosurgical procedures as well as microwave
ablation procedures and vessel sealing procedures. The generator may
include a plurality of outputs for interfacing with various
electrosurgical instruments (e.g., a monopolar active electrode, return
electrode, bipolar electrosurgical forceps, footswitch, etc.). Further,
the generator includes electronic circuitry configured for generating
radio frequency power specifically suited for various electrosurgical
modes (e.g., cutting, blending, division, etc.) and procedures (e.g.,
monopolar, bipolar, vessel sealing).

[0025]FIG. 1A is a schematic illustration of a monopolar electrosurgical
system according to one embodiment of the present disclosure. The system
includes an electrosurgical instrument 2 having one or more electrodes
for treating tissue of a patient P. The instrument 2 is a monopolar type
instrument including one or more active electrodes (e.g., electrosurgical
cutting probe, ablation electrode(s), etc.). Electrosurgical RF energy is
supplied to the instrument 2 by a generator 20 via an supply line 4,
which is connected to an active terminal 30 (FIG. 2) of the generator 20,
allowing the instrument 2 to coagulate, ablate and/or otherwise treat
tissue. The energy is returned to the generator 20 through a return
electrode 6 via a return line 8 at a return terminal 32 (FIG. 2) of the
generator 20. The active terminal 30 and the return terminal 32 are
connectors configured to interface with plugs (not explicitly shown) of
the instrument 2 and the return electrode 6, which are disposed at the
ends of the supply line 4 and the return line 8, respectively.

[0026] The system may include a plurality of return electrodes 6 that are
arranged to minimize the chances of tissue damage by maximizing the
overall contact area with the patient P. In addition, the generator 20
and the return electrode 6 may be configured for monitoring so-called
"tissue-to-patient" contact to insure that sufficient contact exists
therebetween to further minimize chances of tissue damage.

[0027]FIG. 1B is a schematic illustration of a bipolar electrosurgical
system according to the present disclosure. The system includes a bipolar
electrosurgical forceps 10 having one or more electrodes for treating
tissue of a patient P. The electrosurgical forceps 10 includes opposing
jaw members having an active electrode 14 and a return electrode 16
disposed therein. The active electrode 14 and the return electrode 16 are
connected to the generator 20 through cable 18, which includes the supply
and return lines 4, 8 coupled to the active and return terminals 30, 32,
respectively (FIG. 2). The electrosurgical forceps 10 is coupled to the
generator 20 at a connector 21 having connections to the active and
return terminals 30 and 32 (e.g., pins) via a plug disposed at the end of
the cable 18, wherein the plug includes contacts from the supply and
return lines 4, 8.

[0028] The generator 20 includes suitable input controls (e.g., buttons,
activators, switches, touch screen, etc.) for controlling the generator
20. In addition, the generator 20 may include one or more display screens
for providing the user with variety of output information (e.g.,
intensity settings, treatment complete indicators, etc.). The controls
allow the user to adjust power of the RF energy, waveform parameters
(e.g., crest factor, duty cycle, etc.), and other parameters to achieve
the desired waveform suitable for a particular task (e.g., coagulating,
tissue sealing, intensity setting, etc.). The instrument 2 may also
include a plurality of input controls that may be redundant with certain
input controls of the generator 20. Placing the input controls at the
instrument 2 allows for easier and faster modification of RF energy
parameters during the surgical procedure without requiring interaction
with the generator 20.

[0029]FIG. 2 shows a schematic block diagram of the generator 20 having a
controller 24, a high voltage DC power supply 27 ("HVPS") and an RF
output stage 28. The HVPS 27 is connected to a conventional AC source
(e.g., electrical wall outlet) and provides high voltage DC power to an
RF output stage 28, which then converts high voltage DC power into RF
energy and delivers the RF energy to the active terminal 30. The energy
is returned thereto via the return terminal 32.

[0030] In particular, the RF output stage 28 generates sinusoidal
waveforms of high RF energy. The RF output stage 28 is configured to
generate a plurality of waveforms having various duty cycles, peak
voltages, crest factors, and other suitable parameters. Certain types of
waveforms are suitable for specific electrosurgical modes. For instance,
the RF output stage 28 generates a 100% duty cycle sinusoidal waveform in
cut mode, which is best suited for ablating, fusing and dissecting tissue
and a 1-25% duty cycle waveform in coagulation mode, which is best used
for cauterizing tissue to stop bleeding.

[0031] The generator 20 may include a plurality of connectors to
accommodate various types of electrosurgical instruments (e.g.,
instrument 2, electrosurgical forceps 10, etc.). Further, the generator
20 is configured to operate in a variety of modes such as ablation,
monopolar and bipolar cutting coagulation, etc. It is envisioned that the
generator 20 may include a switching mechanism (e.g., relays) to switch
the supply of RF energy between the connectors, such that, for instance,
when the instrument 2 is connected to the generator 20, only the
monopolar plug receives RF energy.

[0032] The controller 24 includes a microprocessor 25 operably connected
to a memory 26, which may be volatile type memory (e.g., RAM) and/or
non-volatile type memory (e.g., flash media, disk media, etc.). The
microprocessor 25 includes an output port that is operably connected to
the HVPS 27 and/or RF output stage 28 allowing the microprocessor 25 to
control the output of the generator 20 according to either open and/or
closed control loop schemes. Those skilled in the art will appreciate
that the microprocessor 25 may be substituted by any logic processor
(e.g., control circuit) adapted to perform the calculations discussed
herein.

[0033] A closed loop control scheme is a feedback control loop wherein
sensor circuitry 22, which may include a plurality of sensors measuring a
variety of tissue and energy properties (e.g., tissue impedance, tissue
temperature, output current and/or voltage, voltage and current passing
through the tissue, etc.), provides feedback to the controller 24. Such
sensors are within the purview of those skilled in the art. The
controller 24 then signals the HVPS 27 and/or RF output stage 28, which
then adjust DC and/or RF power supply, respectively. The controller 24
also receives input signals from the input controls of the generator 20
or the instrument 2. The controller 24 utilizes the input signals to
adjust power outputted by the generator 20 and/or performs other control
functions thereon.

[0034] The present disclosure provides for a system and method for
monitoring electrosurgical procedures using one or more tissue
parameters, which include real impedance, imaginary impedance, phase
angle, voltage, current, average power and combinations thereof. The use
of tissue parameters to control delivery of electrosurgical energy is
discussed with respect to performing ablation procedures, however, those
skilled in the art will appreciate that the illustrated embodiments may
be utilized with other electrosurgical procedures and/or modes.

[0035] In embodiments, various tissue parameters may be measured, recorded
and then plotted to form tissue parameter plots. The tissue parameter
plots are then filtered to obtain a filtered curve that correlates to the
size of the ablation volume. In particular, FIG. 3 shows a plot 300 of an
treatment volume calculated based on temperature measurements. The
treatment volume may be estimated using an Arrhenius ablation model
implemented via LABVIEW® software, which is available from National
Instruments of Austin, Tex. The software may be executed on a variety of
computing devices, such as Luxtron thermal probes available from
LumaSense Technologies of Santa Clara, Calif., that may also interface
with a plurality of suitable temperature measurement devices that provide
temperature measurements over a predetermined period of time (e.g., about
15 minutes) to the computing device. The probes may be disposed at
multiple locations to provide for different temperature measurements
which allows for extrapolation of the size of the ablation volume. The
software then calculates the estimated treatment volume based on the
models discussed above that are implemented in the software. In
embodiments, the size of the ablation volume may also be determined by
excising the volume, obtaining a plurality of slices of the ablation
volume and measuring the cross-sectional size of the ablation volume of
each of the slices. This procedure may be performed to verify the
accuracy of the modeled treatment volume as determined by the Arrhenius
ablation model.

[0036] FIGS. 4-8 illustrate plots of various tissue parameters. FIG. 4
shows a plot 400 of reactive impedance. Plot 400 is obtained by
normalizing a plot of imaginary (e.g., reactive) impedance vs. time. The
imaginary impedance values are filtered prior to normalization, which may
be accomplished by assigning 1 to an ending value of the plot 400 and 0
to the starting value. The plot 400 may be smoothed by convolution and
the peaks may then be detected by using an extrema function. Connecting
the peaks of the plot 400 provides for a correlated plot 402, which
substantially matches the shape of the treatment volume plot 300. This
may be accomplished by curve-fitting using a spline function and then
correlating the two plots 300 and 402. These functions may be performed
by using MATLAB® environment, which provides for convolution, extrema,
curve fitting, and correlation functions, available from Mathworks of
Natick, Mass. In particular, the correlation value, ρ, for the plot
402 with the plot 300 was about 1, which denotes a high degree of
correlation.

[0037] Similar correlation is also illustrated by FIGS. 5-8. In
particular, FIG. 5 shows a plot 500 of phase angle measurements, which is
also normalized. Connecting the peaks of the plot 500 provides for a
correlated plot 502, which when inverted substantially matches the shape
of the plot 300. FIGS. 6-8 show a plot 600 of real impedance, a plot 700
of voltage, and a plot 800 of average power, respectively, all of which
are scaled. Connecting the peaks of the plot 600 provides for a
correlated plot 602, which when inverted also substantially matching the
plot 300. The plots 702 and 802 may be generated based on ripples found
at about the midpoint of the leading rising or falling edge of each of
the peaks of the plots 700 and 800, respectively. Ripples may be
identified as any fluctuations in the peak aside from the peak itself.
The resulting plots are also inverted to provide for the correlated plots
702 and 802.

[0038] The relationship between the plots 402, 502, 602, 702, 802 and 300
illustrates the correlation between various tissue parameters, such as
reactive impedance, phase angle, real impedance, voltage and average
power and the size of the ablation as determined using temperature
measurements.

[0039] Similar to the correlation value of the plot 402 with the plot 300,
the correlation value for the plots 502 and 602 with the plot 300 was
also about 1. This illustrates, that imaginary impedance, real impedance
and phase angle yield patterns that are highly correlated with treatment
volume dynamics and are suitable for detecting process progression and
possible trigger points for initiating procedure termination. Although
each of the tissue parameters appears to be correlated to the treatment
volume (i.e., ablation volume), while not wishing to be bound by theory,
it is believed that each of the tissue parameters may be measuring
different characteristics of tissue consistency.

[0040] Complex impedance consists of real and imaginary impedance. Real
impedance is identified with resistance and imaginary impedance is
identified with reactance. In addition, reactive impedance may be either
inductive or capacitive. Purely resistive impedance exhibits no phase
shift between the voltage and current, whereas reactance induces a phase
shift θ between the voltage and the current passing through the
tissue, thus imaginary impedance may be calculated based on the phase
angle or phase shift between the voltage and current waveforms.

[0041] Changes in the imaginary impedance during energy delivery may be
used as an indication of changes in tissue properties due to energy
application. More specifically, imaginary impedance may be used to detect
the formation of micro bubbles, bubble fields and tissue desiccation that
impart an electrical reactivity to the tissue that corresponds to sensed
imaginary impedance. The tissue reactivity is reflective of the energy
that is being delivered into the tissue. Thus, the measured change in
imaginary impedance may be used as an indication of the amount of energy
resident in the tissue. Monitoring of the resident energy in combination
with monitoring of the energy being supplied by the generator allows for
calculation of energy escaping the tissue during treatment, thereby
allowing for determination of efficiency of the treatment process as well
as any inadvertent energy drains.

[0042] As the ablation volume increases, so does the region of tissue that
can support formation of micro bubbles. The presence of micro bubbles in
soft tissue increases the capacitance of the dielectric character of the
affected tissue. As the energy being applied to the tissue increases, the
micro bubbles then accrete to form macro bubbles, which decreases the
capacitance but increases real impedance of the tissue. Hence, the shift
of bubble population from micro to macro levels is indicated by a shift
of measured impedance from reactive to real. One consequence of this is
that water content of the tissue is displaced by this transformation and
that displaced water may be harnessed to create desired tissue effects
(e.g., tissue division).

[0043]FIG. 9 shows a method for controlling output of the generator 20
based on various tissue parameters. The method may be embodied as a
software application embedded in the memory 26 and executed by the
microprocessor 25 to control generator 20 output based on measured tissue
parameters or changes thereof as a function of time. In step 200,
ablation energy is delivered into tissue and various tissue parameters
are measured by the sensor circuitry 22. In particular, the sensor
circuitry 22 measures tissue and energy parameters based on voltage and
current waveforms passing through the tissue and determines voltage,
current, average power, phase angle between the waveforms, real
impedance, and imaginary impedance (e.g., the imaginary component of the
complex impedance) based on the phase angle between the waveforms.

[0044] In step 202, the tissue and energy parameters are measured and are
plotted in real-time to generate a tissue or energy parameter plot as
shown in FIGS. 4-8. The plot is pre-filtered to allow for faster
processing to generate a pre-filtered curve having smoother curves.
Various filters may be utilized to achieve the pre-filtered curve, such
as Kalman Filter and the like. The plot is also normalized as discussed
above, thereafter, the peaks are detected and are interconnected to
produce a correlated plot as shown in FIG. 4. The peaks are detected by
the generator 20 by recording an amplitude value of a specific tissue or
energy parameter (e.g., reactive impedance) corresponding to the peak.
The peaks may be identified by tracking the changes in the rate of
change, e.g., slope of the plotted tissue or energy parameter plot, e.g.,
plot 400.

[0045] In embodiments, as shown in FIGS. 5 and 6, the plot may also be
inverted to correlate with the treatment volume plot. In further
embodiments, as shown in FIGS. 7 and 8, the plot may be generated based
on the ripples of each of the peaks. The ripples on the rising edge in
both voltage and average power plots 700 and 800, respectively are due to
the energy being pulsed. The ripples may be detected based on radical
changes in the slope (e.g., rapid oscillations between positive and
negative values during a rising edge). The rising or falling edge may
also be identified by tracking a positive or negative slope,
respectively, for a predetermined period of time. The generator 20 then
records the amplitude values of the tissue or energy parameter
corresponding to ripples and generates a plot therethrough. In
embodiments, the plot may also be inverted.

[0046] As the curve is generated, it may be analyzed to determine a
shut-off point. As discussed above, as energy is applied to the tissue,
micro bubbles form in the intracellular and intercellular space,
resulting in a low starting imaginary impedance (e.g., more negative,
associated with more inductive). As the temperature of the tissue
increases, liquid water is driven away from the tissue regions close to a
phase transition temperature (e.g., 80° C. and above), more micro
bubbles form, steam bubbles increase in size and these regions become
desiccated. The desiccated regions of tissue have higher impedance and
therefore contribute to the capacitive impedance. These phenomena are
mostly reversible because as the temperature increases and drives the
water out, osmotic pressures generate a reverse flow of water. As a
result, the tissue seeks a new equilibrium or steady state condition
between a desiccated state and a hydrated state to reestablish energy
balance.

[0047] Once the equilibrium is achieved, the thermal kill zone (e.g.,
treatment volume) does not grow significantly. Thus, establishment of
equilibrium correlates to the maximum thermal kill zone and may be used
to determine whether termination of energy application is appropriate. In
other words, monitoring of imaginary impedance allows for determination
of the equilibrium, which correlates with the maximum thermal kill zone
and may therefore, serve as a suitable threshold of intelligent shut-off.

[0048] Determination of the equilibrium may be determined by analyzing the
slope of the tissue parameter curve or the rate of change of the
imaginary impedance. The determination of the slope may be performed at
the sensor circuitry 22 and/or the controller 24. A slope of about 0 is
believed to be reflective of the establishment of equilibrium, whereas a
negative slope corresponds to reduction in energy accumulation within the
tissue. Prior to slope analysis, the tissue parameter curve is filtered
using a single pole recursive filter. Thus, the first filter smoothes out
the impedance curve 110 and the recursive filtering detects direction and
magnitude of the slope changes as described below.

[0049] In step 204, the slope of the tissue parameter curve (e.g., rate of
change of the tissue parameter) is determined. According to one
embodiment of the present disclosure, the determination of the rate of
change may be achieved via single pole recursive filtering that averages
a predetermined number of tissue parameter values to achieve the rate of
change value. Any number of impedance filters may be used and are based
on the following formula (I):

ZfXn=Zin*A+ZfXn-1*B (1)

[0050] A and B are dependent on a time constant and may be specified by
the user, via the input controls of the generator 20, for each particular
impedance filter ZfX. When calculating A and B, the following formulas
may be used:

B=e (-1/number of samples);

A=1-B.

[0051] The sample rate may also be specified by the user for calculating
the number of samples. In formula (I), Zin is the new root mean square
tissue parameter value (e.g., ZiRMS) just calculated, and
ZfXn-1 is the filtered tissue parameter, for the filter number
specified by X, from the previous iteration through the loop, and
ZfXn is the new filtered impedance value for the filter number
specified by X. In one embodiment, the sample rate for calculating the
number of samples may be synchronized with the loop time of the
microprocessor 25. Accordingly, within about 5 time constants, the final
output of the tissue parameter filter may be provided that corresponds to
the slope of the tissue parameter curve. In another embodiment, an
initial base tissue parameter may be used to preload the tissue parameter
filters.

[0052] In step 206, the slope of the tissue parameter curve is analyzed.
In one embodiment, the slope is analyzed using three regions (e.g., two
thresholds). In step 208, it is determined whether the slope is above a
first predetermined threshold (e.g., a positive threshold number). In
step 210 it is determined whether the slope is between the first
threshold and a second predetermined threshold (e.g., a negative number).
In step 212, it is determined if the slope is below the second threshold.
In another embodiments, a plurality of regions may be utilized based on
multiple actions that need to be performed in response to varying slope
values. Based on the analysis of the rate of change of the tissue
parameter (e.g., slope) and/or the tissue parameter, the controller 24
adjusts the output of the generator 20 as discussed in more detail below.

[0053] When the slope is above the first threshold, this indicates that
the thermal profile is growing and that energy application may continue
in step 208. The process then reverts to step 206 to continue slope
monitoring and energy application. When the slope is between the first
and second thresholds, the thermal profile is in equilibrium which
denotes that equilibrium has been reached and an intelligent shut-off
process is commenced as shown in step 214. Once it is determined that
equilibrium has been reached, a verification is made if a predetermined
time delay has expired. This provides a second verification to determine
that a substantial portion of the tissue has been treated. The time delay
may be user-selectable either by entering a predetermined time value or
by selecting one of proposed delay periods. In one embodiment, one of the
options may be a time delay corresponding to the shortest time for
establishing termination of the procedure and another option may be a
time delay corresponding to a conservative treatment regimen that
assurance 100% cell kill ratio.

[0054] In one embodiment, an intermediate time delay may also be utilized.
An intermediate time delay is triggered in step 216 once an equilibrium
is reached and the slope detection still continues to make sure that the
slope trends do not change. If the slope increases above the first
threshold, then energy application resumes. At this point, the
intermediate time delay is triggered and slope interrogation continues.
In other words, the process then reverts to step 206 to continue slope
monitoring and energy application.

[0055] When the slope is less than the second threshold, this denotes that
energy application efficiency is decreasing and the procedure should be
terminated. This may be caused by proximity to a blood vessel and other
obstructions. Upon encountering negative slopes that are below the second
threshold, the process in step 218 terminates the application of energy
and/or alerts the user of the decrease in energy application.

[0056] While several embodiments of the disclosure have been shown in the
drawings and/or discussed herein, it is not intended that the disclosure
be limited thereto, as it is intended that the disclosure be as broad in
scope as the art will allow and that the specification be read likewise.
Therefore, the above description should not be construed as limiting, but
merely as exemplifications of particular embodiments. Those skilled in
the art will envision other modifications within the scope and spirit of
the claims appended hereto.